1. Field of the Invention
The present invention relates to elastic wave filter devices that are used as, for example, band-pass filters for mobile telephones or other suitable uses, and, more particularly, to a longitudinally-coupled-resonator-type elastic wave filter device.
2. Description of the Related Art
Various surface acoustic wave filter devices have been used as band-pass filters for mobile telephones. For example, Japanese Patent No. 3520420 discloses a surface acoustic wave filter device used as a band-pass filter for a mobile telephone, and having a balanced-to-unbalanced conversion function. The balance between a pair of balanced terminals is improved by using an IDT 1001 illustrated in FIG. 19.
The IDT 1001 is disposed on a piezoelectric substrate 1000. The IDT 1001 includes electrode fingers 1002 and 1003 near one end thereof in a direction in which a surface acoustic wave propagates. The electrode fingers 1002 and 1003 are apodization-weighted electrode fingers. The electrode fingers 1002 and 1003 are connected to a ground potential and a hot-side potential, respectively. The length of the electrode fingers 1002 and 1003 is less than that of the other electrode fingers 1004 connected to the ground potential and the other hot-side electrode fingers 1005.
On the other hand, in a portion in which the electrode fingers 1002 and 1003 are provided, a dummy electrode 1006 is disposed for series weighting. The dummy electrode 1006 has a first dummy electrode finger portion 1006a, a second dummy electrode finger portion 1006b, and a connecting portion 1006c connecting the first dummy electrode finger portion 1006a and the second dummy electrode finger portion 1006b. 
The first dummy electrode finger portion 1006a is disposed along the extension line of the longitudinal axis of the electrode finger 1002 with a gap between it and the forward end of the electrode finger 1002 so that it overlaps with the electrode finger 1003 in the direction of propagation of a surface acoustic wave. The second dummy electrode finger portion 1006b is similarly disposed along the extension line of the longitudinal axis of the electrode finger 1003 with a gap between it and the forward end of the electrode finger 1003 so that it overlaps, that is, interdigitates with the electrode finger 1002 in the direction of propagation of a surface acoustic wave. The connecting portion 1006c extends in a region between the forward ends of the electrode fingers 1002 and 1003 in the direction of propagation of a surface acoustic wave, and connects the inner ends of the first dummy electrode finger portion 1006a and the second dummy electrode finger portion 1006b. 
In a portion in which the dummy electrode 1006 is disposed, excitation of a surface acoustic wave is weakened by the dummy electrode 1006. By disposing the dummy electrode 1006 in an IDT connected to one of a pair of balanced terminals at which a surface acoustic wave is excited at a higher level than that of excitation of a surface acoustic wave at the other one of the balanced terminals, the surface acoustic wave excitation levels at the balanced terminals are relatively close to each other. Consequently, the balance between the balanced terminals is improved.
As described previously, by performing series weighting on an IDT in a longitudinally-coupled-resonator-type surface acoustic wave filter device having the balanced-to-unbalanced conversion function, the balance between the balanced terminals can be improved.
However, if such a series-weighed IDT is used in a longitudinally-coupled-resonator-type surface acoustic wave filter device, a high level of ripple is generated in a pass band.